Artificial joint prostheses are widely used today, restoring mobility to patients affected by a variety of conditions, particularly arthritis. The satisfactory performance of these devices can be affected not only by the design of the component itself, but also by the long term wear resistance of the bearing components. Inadequate wear resistance can lead to the generation of harmful debris; continued wear can ultimately lead to component failure due to wear through, fracture, loosening, or dislocation of one or more of the involved implants.
Ultrahigh molecular weight polyethylene (hereinafter referred as “UHMWPE”) has proven to be a useful material for orthopaedic implants that function as bearing components in articulating joints. However, UHMWPE wear debris, if present in excess, can result in lysis of the bone tissue that surrounds and supports the implant, which can in turn lead to the need for surgical revision. Many methods of improving the wear resistance of the UHMWPE used in orthopaedic implants have been proposed and implemented with varying degrees of promise for the long-term survival of the reconstructed joints.
One method to improve the wear resistance of UHMWPE is to increase the degree of crosslinking of the polymer using ionizing radiation, such as gamma ray or electron beam irradiation. Ionizing radiation results in the breaking of carbon-carbon and/or carbon-hydrogen bonds, and the formation of free radicals, at the molecular level of the polymer. In the absence of oxygen, these free radicals can form carbon-carbon crosslinks between adjacent molecules, returning to a stable molecular structure. This stabilization process can be accelerated by elevating the temperature of the irradiated UHMPWE, below, near, or above the melt temperature of the UHMWPE. Based on numerous wear studies from multiple laboratories, the wear resistance of UHMWPE generally increases as the degree of crosslinking of the polymer increases.
However, if oxygen is present during or after irradiation, and the free radicals have not been stabilized by annealing or otherwise, the free radicals can result in oxidative chain scission; during oxidative chain scission, oxygen reacts with a free radical, breaking a polymer chain, and generating a new free radical. This process is hereinafter referred to as “oxidation”. Oxidation of UHMWPE can be detrimental to the mechanical properties and wear resistance of the material.
The production of medical devices from UHMWPE is generally done by either machining of preforms, such as bars, plates, or rods, or direct compression molding of parts from UHMWPE powder. Machining can also be applied to molded parts to produce the finished geometry. After the parts are formed, and other steps such as cleaning and inspection are performed as appropriate, the parts are packaged; for many devices, such as orthopaedic joint replacement implants, the parts are sterilized after packaging using any of several suitable methods. Such methods include both ionizing irradiation and non-ionizing methods such as ethylene oxide and gas plasma sterilization.
For these reasons, various methods of irradiating and storing the UHMWPE, which may include heat treatment or other stabilization methods, have been developed to increase the crosslink density while minimizing oxidation of the polymer. Some of these methods include application of ionizing radiation to a preform, such as a bar, plate, or rod, and the application of an elevated temperature during and/or after irradiation, but prior to machining the parts from the treated preform. Other methods include packaging the formed implant devices prior to treatment, and then irradiating the packaged devices either as part of, or in addition to, the sterilization process. These irradiated devices may then be heat treated to stabilize the material.
Since, in some of these methods, the heat treatment is performed on finished parts, it must be applied in such a manner that there is negligible risk of dimensional deviations of the finished parts from the part specifications, so that functionality of the finished parts is not compromised. This can be especially important for parts that, in use, connect or otherwise interact with other parts, such as in modular assemblies, where proper function is dependent on the precise fit of the parts together. When irradiation and/or heat treatment is done on packaged parts, such as when irradiation is used for sterilization, the ability to inspect the parts for dimensional specifications or other physical conformance criteria after irradiation and/or after heat treatment is greatly limited. For this reason, both the irradiation and heat treatment of packaged parts must be done such that dimensional changes, such as deformation or shrinkage of the parts, are minimal. This may require limiting the radiation dose to a dose that is less than optimal for wear performance, since UHMWPE can shrink in response to ionizing radiation. To limit part deformation to acceptable levels, post-packaging heat treatment may require using time and temperature profiles with maximum sustained temperatures significantly below the deformation temperature of UHMWPE (below 80° C.), and corresponding heat treatment time periods exceeding 48 hours, to achieve adequate stabilization of the material.
The fabrication of UHMWPE parts by machining can result in residual stresses in the UHMWPE. These residual stresses can result in deformation of the parts after machining. This can lead to some parts deviating from the design specifications some time after machining, even if the parts met the design specifications immediately after machining. Part designs that include thin walls, complex geometries, and/or small geometric tolerances, for example, can be particularly problematic. The relaxation of residual stresses is accelerated by the application of elevated temperatures. However, if the irradiated preforms are heat treated prior to machining of the preforms, it may take longer for the stress relaxation process to complete. For parts that are heat treated after final packaging, machining techniques to minimize residual stresses must be employed, or the ability to produce parts with certain specifications may be limited.